Secondary batteries such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide range of applications. A secondary battery generally includes battery members such as electrodes (positive electrode and negative electrode) and a separator that isolates the positive electrode and the negative electrode from one another and prevents short-circuiting between the positive and negative electrodes.
A battery member that includes a porous membrane as a protective layer may be conventionally used in a secondary battery with the aim of improving heat resistance and strength.
Specifically, the porous membrane may be a membrane that is formed by binding inorganic particles with a binder. A porous membrane of this type is normally formed by preparing a slurry composition (hereinafter, also referred to as a “composition for a porous membrane”) containing inorganic particles and a binder dispersed in a dispersion medium, applying this composition for a porous membrane onto a separator substrate or an electrode substrate obtained by forming an electrode mixed material layer on a current collector, and then drying the composition for a porous membrane.
In recent years, there has been increased interest in the use of aqueous compositions for porous membranes (i.e., compositions in which an aqueous medium serves as a dispersion medium) in the production of porous membranes from viewpoints such as reduction of environmental impact. For example, PTL 1 reports a technique for forming a porous membrane having optimal porosity, excellent uniformity, and thermal stability using a composition for a porous membrane that contains a binder and an inorganic oxide powder having specific properties dispersed in a dispersion medium such as water.